Unsaturated thermosetting resins such as polyesters and vinyl esters have, when combined with reinforcements such as glass fibres and cured, good mechanical properties. This has led to the wide acceptance of glass reinforced plastics (GRP) in such diverse applications as, for example, boats, pipes, tanks, car components and building panels.
However they could be improved if the toughness of the resin matrix could be raised so that they did not fail in a brittle mode when subjected to sudden or excessive stress. In this respect they are particularly notch sensitive i.e. they fail due to cracks propagating from small flaws or inclusions within the material or from small nicks on the surface.
Other plastics materials, both thermoset and thermoplastic, suffer from the same defect and it is known for example to improve the toughness (e.g. resistance to impact) of polystyrene by the incorporation of a rubbery component especially as a separate dispersed phase within the polymer which is the continuous phase. By this means fracture energy is increased without significant reduction of other mechanical properties such as modulus or ultimate strength.
The means of forming such a discrete particulate rubbery phase in many polymers including polystyrene, styrene/acrylonitrile copolymers and epoxide resins is well known. Some involve dissolving a suitable elastomer which may be a solid or liquid rubber in the monomer(s) which are subsequently polymerised by a free radical mechanism. In the case of epoxide resins the elastomeric phase is dissolved in the resin which is then cured by the addition of known curing agents with heating if necessary (see G.B. No. 1 408 798 and Paul et al, Polymer 1977, Vol. 18, pages 945-950).
During polymerisation or curing, depending on the polymer to be toughened, the elastomer becomes incompatible with the polymerising matrix and separates as a particulate phase. In the case of thermoplastics such as polystyrene the elastomer becomes a particulate phase through stirring during the polymerisation. With a thermoset system such as an epoxide resin it is necessary to select an elastomer which separates during cure into a particulate phase without agitation. During polymerisation or cure some chemical reactions can occur between the elastomer and the matrix so that the elastomer becomes grafted or cross-linked by the matrix. The amounts of elastomer used can be varied over wide limits but is commonly in the range 5-10% by weight of the total composition.
Attempts have been made to incorporate elastomers within unsaturated polyesters and similar resins but these have only shown minimal or insignificant improvements in fracture energy. They have usually employed elastomers similar to those successfully used with epoxide resins which are low molecular weight polymers or copolymers of butadiene, especially butadiene/acrylonitrile copolymers, with functional terminal groups, normally hydroxyl or carboxyl.
Unfortunately these liquid rubbers have poor solubility in such unsaturated resins so that if the rubber concentration is above about 3% by weight the rubber tends to separate before the resin is cured and forms large globules within the resin or a pool of rubber on the surface. The result is that on curing large rubber particles are formed which do not contribute to the toughness of the composite. If a rubber of greater solubility is used, e.g. hydroxy terminated polyepichlorhydrin, it may not separate into a particulate phase on curing, gives only a very little improvement in toughness, and impairs other desirable mechanical properties.
The importance of solubility in the liquid and curing resins systems can be explained by the fact that polyesters become rigid at only about 20% conversion (as measured by disappearance of double bonds) whereas epoxide resins become rigid at much higher conversions (as measured by disappearance of oxirane groups). For this reason an elastomer must separate into a particulate phase when used with unsaturated resin at a lower conversion than when used with an epoxide resin.